Deck footing size calculator

What it counts

Tributary area is the patch of deck one post holds up: half the joist span, multiplied by the post spacing along the beam. Multiply that by the load per square foot and you have the pounds landing on one footing.

Divide by the soil's presumptive bearing capacity and you get the bearing area the footing needs. Convert that circle to a diameter and round up to something you can actually dig.

Every step is arithmetic. The only judgement is the soil class, and that is not yours to judge — it is assigned by the building department, and it can require a soils investigation on a difficult lot.

Worked example

A 12-foot joist span with posts at 6-foot centres puts a 6 × 6 = 36 square foot patch on each footing. At the residential 40 + 10 psf that is 1,800 lb. On a 2,000 psf soil the bearing area needed is 0.9 square feet — a circle of roughly 13 inches, rounded up to 14.

Widen the post spacing to 8 feet and the same beam puts 48 square feet on each post: the footing grows, and because area goes with the square of the diameter, it grows less than you would guess. That is often the right trade, and the beam tool is where you make it.

Deck footing size chart

The code publishes the same answer as a lookup table: Table R507.3.1 in IRC 2021 R507.3. Below are its 40 psf live load rows. For the worked example, 36 square feet on 2,000 psf soil falls between the 20 and 40 square foot rows, so the table asks for a round footing of about 13 inches; the calculator above rounds up to the next even inch and says 14. Heavier snow loads have their own rows in the table, with bigger footings.

The table allows interpolation between rows but not extrapolation past 160 square feet, and its thickness minimums apply only to plain concrete footings.

Depth is a different number

Nothing on this page tells you how deep to dig. Depth is set by the local frost line and by the requirement that the footing bear on undisturbed soil, and both are local. Get it from your building department's deck handout before you rent an auger — the full argument is in footing depth.

Sources

• 2021 IRC § R507.3 and Table R507.3.1, Deck Footings (UpCodes) read 2026-10-18
• 2021 IRC Table R401.4.1, Presumptive Load-Bearing Values of Foundation Materials (UpCodes) read 2026-10-18